[sdiy] MPF102 sub

[Ian Fritz]

In days of old the 2n3819 was used as a near equivalent to the MPF102.  It 
is still manufactured (Central Semi) and is available from Mouser.

Other FETs available from Mouser are:
J105, J107, J109, J111, J112, J113, J310
BF256
2n4392, 2n4393
2n4416
2n5460
2n5484, 2n5485, 2n5486.

Things to remember:  Just because a certain substitution can be made in one 
project doesn't mean it can be made in all projects.  When you purchase a 
bunch you usually get pinchoff voltages near the "typical" value, which may 
be much smaller than the max value.  If you need a low pinchoff you can 
always hope that works, but in general you should measure the pinchoff 
voltage to be sure.  Do not substitute a MOSFET unless you know what you 
are doing.  Most n-channel types are enhancement-mode devices, meaning that 
the bias requirements are different.  Additionally, nearly all have 
protection diodes, which means they cannot switch bipolar signals. Plus, 
they have large input capacitance unless you track down the few devices 
that are suitable for low-power switching. This capacitance can put huge 
switching transients on the signal, so that for discharging a VCO cap the 
output is very messy.

I've pretty much decided that for future designs I'm going to use the IC 
switches (dg211, etc.)  Even though they are expensive, there are no 
worries about pinchoff voltage variations, and I doubt they are going to 
vanish anytime soon.

Ian